To estimate the US national prevalence of tibiofemoral radiographic knee osteoarthritis (RKOA) with and without symptoms, and its influence on functional tasks. Radiographic and interview data from the National Health and Nutrition Examination Survey (NHANES III), a nationally representative cross-sectional health examination survey, were used to estimate lifetime RKOA prevalence in adults age 60 years and older. Demographic trends, self-reported activity limitations, physical performance test results, and patterns of recent analgesic use were analyzed. Among US adults, the prevalence of RKOA and symptomatic RKOA was 37.4% and 12.1%, respectively. RKOA prevalence was greater among women than men (42.1% vs 31.2%). Women had significantly more Kellgren-Lawrence Grade 3-4 changes (12.9% vs 6.5% in men). However, symptomatic RKOA prevalence did not differ by sex. Additionally, some 1.6% of US adults had knee joint replacement. Multivariable analysis showed significantly higher odds of both RKOA and symptomatic RKOA with greater body mass index (BMI > or = 30), greater age, non-Hispanic Black race/ethnicity, and among men with manual labor occupations. Only symptomatic RKOA was significantly associated with self-reported activity limitations: difficulty walking, stooping, standing from a seated position, and stair climbing. Adults with symptomatic RKOA used significantly more assistive walking devices, had slower measured gait velocities, and used significantly more prescription nonsteroidal antiinflammatory drugs and prescription narcotics, and nonprescription acetaminophen. NHANES III data provide an overall national assessment of the prevalence, demographic distributions, and functional impact of symptomatic knee OA, which affects more than 1 in 10, or 4.3 million older US adults.